!
!  - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
!  SLEPc - Scalable Library for Eigenvalue Problem Computations
!  Copyright (c) 2002-2021, Universitat Politecnica de Valencia, Spain
!
!  This file is part of SLEPc.
!  SLEPc is distributed under a 2-clause BSD license (see LICENSE).
!  - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
!
!  Program usage: mpiexec -n <np> ./ex1f [-help] [-n <n>] [all SLEPc options]
!
!  Description: Simple example that solves an eigensystem with the EPS object.
!  The standard symmetric eigenvalue problem to be solved corresponds to the
!  Laplacian operator in 1 dimension.
!
!  The command line options are:
!    -n <n>, where <n> = number of grid points = matrix size
!
! ----------------------------------------------------------------------
!
      program main
#include <slepc/finclude/slepceps.h>
      use slepceps
      implicit none

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!     Declarations
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!
!  Variables:
!     A     operator matrix
!     eps   eigenproblem solver context

      Mat            A
      EPS            eps
      EPSType        tname
      PetscReal      tol, error
      PetscScalar    kr, ki
      Vec            xr, xi
      PetscInt       n, i, Istart, Iend
      PetscInt       nev, maxit, its, nconv
      PetscInt       col(3)
      PetscInt       i1,i2,i3
      PetscMPIInt    rank
      PetscErrorCode ierr
      PetscBool      flg
      PetscScalar    value(3)

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!     Beginning of program
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      call SlepcInitialize(PETSC_NULL_CHARACTER,ierr)
      call MPI_Comm_rank(PETSC_COMM_WORLD,rank,ierr)
      n = 30
      call PetscOptionsGetInt(PETSC_NULL_OPTIONS,PETSC_NULL_CHARACTER,  &
     &                        '-n',n,flg,ierr)

      if (rank .eq. 0) then
        write(*,100) n
      endif
 100  format (/'1-D Laplacian Eigenproblem, n =',I3,' (Fortran)')

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!     Compute the operator matrix that defines the eigensystem, Ax=kx
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      call MatCreate(PETSC_COMM_WORLD,A,ierr)
      call MatSetSizes(A,PETSC_DECIDE,PETSC_DECIDE,n,n,ierr)
      call MatSetFromOptions(A,ierr)
      call MatSetUp(A,ierr)

      i1 = 1
      i2 = 2
      i3 = 3
      call MatGetOwnershipRange(A,Istart,Iend,ierr)
      if (Istart .eq. 0) then
        i = 0
        col(1) = 0
        col(2) = 1
        value(1) =  2.0
        value(2) = -1.0
        call MatSetValues(A,i1,i,i2,col,value,INSERT_VALUES,ierr)
        Istart = Istart+1
      endif
      if (Iend .eq. n) then
        i = n-1
        col(1) = n-2
        col(2) = n-1
        value(1) = -1.0
        value(2) =  2.0
        call MatSetValues(A,i1,i,i2,col,value,INSERT_VALUES,ierr)
        Iend = Iend-1
      endif
      value(1) = -1.0
      value(2) =  2.0
      value(3) = -1.0
      do i=Istart,Iend-1
        col(1) = i-1
        col(2) = i
        col(3) = i+1
        call MatSetValues(A,i1,i,i3,col,value,INSERT_VALUES,ierr)
      enddo

      call MatAssemblyBegin(A,MAT_FINAL_ASSEMBLY,ierr)
      call MatAssemblyEnd(A,MAT_FINAL_ASSEMBLY,ierr)

      call MatCreateVecs(A,xr,xi,ierr)

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!     Create the eigensolver and display info
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!     ** Create eigensolver context
      call EPSCreate(PETSC_COMM_WORLD,eps,ierr)

!     ** Set operators. In this case, it is a standard eigenvalue problem
      call EPSSetOperators(eps,A,PETSC_NULL_MAT,ierr)
      call EPSSetProblemType(eps,EPS_HEP,ierr)

!     ** Set solver parameters at runtime
      call EPSSetFromOptions(eps,ierr)

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!     Solve the eigensystem
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      call EPSSolve(eps,ierr)
      call EPSGetIterationNumber(eps,its,ierr)
      if (rank .eq. 0) then
        write(*,110) its
      endif
 110  format (/' Number of iterations of the method:',I4)

!     ** Optional: Get some information from the solver and display it
      call EPSGetType(eps,tname,ierr)
      if (rank .eq. 0) then
        write(*,120) tname
      endif
 120  format (' Solution method: ',A)
      call EPSGetDimensions(eps,nev,PETSC_NULL_INTEGER,                 &
     &                      PETSC_NULL_INTEGER,ierr)
      if (rank .eq. 0) then
        write(*,130) nev
      endif
 130  format (' Number of requested eigenvalues:',I2)
      call EPSGetTolerances(eps,tol,maxit,ierr)
      if (rank .eq. 0) then
        write(*,140) tol, maxit
      endif
 140  format (' Stopping condition: tol=',1P,E11.4,', maxit=',I4)

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!     Display solution and clean up
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!     ** Get number of converged eigenpairs
      call EPSGetConverged(eps,nconv,ierr)
      if (rank .eq. 0) then
        write(*,150) nconv
      endif
 150  format (' Number of converged eigenpairs:',I2/)

!     ** Display eigenvalues and relative errors
      if (nconv.gt.0) then
        if (rank .eq. 0) then
          write(*,*) '         k          ||Ax-kx||/||kx||'
          write(*,*) ' ----------------- ------------------'
        endif
        do i=0,nconv-1
!         ** Get converged eigenpairs: i-th eigenvalue is stored in kr
!         ** (real part) and ki (imaginary part)
          call EPSGetEigenpair(eps,i,kr,ki,xr,xi,ierr)

!         ** Compute the relative error associated to each eigenpair
          call EPSComputeError(eps,i,EPS_ERROR_RELATIVE,error,ierr)
          if (rank .eq. 0) then
            write(*,160) PetscRealPart(kr), error
          endif
 160      format (1P,'   ',E12.4,'       ',E12.4)

        enddo
        if (rank .eq. 0) then
          write(*,*)
        endif
      endif

!     ** Free work space
      call EPSDestroy(eps,ierr)
      call MatDestroy(A,ierr)
      call VecDestroy(xr,ierr)
      call VecDestroy(xi,ierr)

      call SlepcFinalize(ierr)
      end

